In recent years, an increasing number of cylinder heads cast from aluminum alloy were used in internal combustion engines. Aluminum alloys have excellent properties of light weight, high thermal conductivity, and superior heat resistance as compared with other light alloys. The aluminum alloys, however, have some problems that they tend to absorb hydrogen gas in molten state and undergo substantial solidification shrinkage. Thus castings often contain casting defects such as pin holes, blow holes, and shrinkage cavities. It is also known that with the slower cooling of molten aluminum alloy, more casting defects occur and even the solidification structure becomes coarser. The mechanical properties of cast aluminum alloys largely depend on the cooling rate during casting and become poor as the cooling rate is reduced.
Cylinder heads for use in internal combustion engines generally have such a large size and a complicated shape that the cooling rate is low during casting. Thus, internal combustion engine cylinder heads cast from aluminum alloys exhibit mechanical properties which are not necessarily sufficient for the above-mentioned reason. Cracks are likely to occur in the combustion chamber-defining wall due to thermal stresses during service.
Thermal stresses induced in internal combustion engine cylinder heads during service are not even over the entirety. Several local areas are highly stressed. Generally, such high load areas include that region interposed beween valve seats of an exhaust port and an intake port, also known as inter-valve region, those regions extended between the valve seats and a port for communication with an auxiliary combustion chamber, that region surrounding the auxiliary combustion chamber communicating port, and that region surrounding an aperture for receiving a spark plug therein. A majority of cracks occur in these regions due to thermal stresses during service. To accommodate particularly high requirements of mechanical properties in these high load regions, the following approaches were made in the prior art.
Modifications were made in molds and cores used in the casting of aluminum alloy in order to improve the mechanical properties of a predetermined portion of the resulting aluminum alloy casting where a high load would be applied. One approach is to increase the wall thickness of the predetermined portion with a design change in size and shape. A second approach is the attachement of a chiller. A sand core is provided with a chiller at a location corresponding to the predetermined portion of the resulting aluminum alloy casting. The cooling rate of the predetermined portion in contact with the chiller is then locally increased. A third approach is by placing a water cooling pipe in a mold in proximity to the predetermined portion of the resulting aluminum alloy casting to locally and forcedly cool the portion.
These prior art approaches have more or less severe problems. The first approach loses the advantages of compactness and light weight characteristic of the use of aluminum alloy because a change of design is required to increased the wall thickness.
In the second approach, the sand core itself becomes very complicated in shape and is thus difficult to mold. In order to cool at the necessary rate internal combustion engine cylinder heads which are large sized castings, the chiller must have a substantially high thermal capacity. In actual practice, however, it is difficult to attach a chiller having such a high thermal capacity. Consequently, the second approach is less effective in improving the quality of castings.
The third approach based on water cooling is truly superior in cooling capacity to the second approach. Nevertheless, the mold itself becomes very complicated in structure. It is very difficult to introduce water to the necessary portion as desired. The attempt is thus unsuccessful in improving the quality of castings to the required extent. In addition, the third approach is difficult to apply to commercial molds from the standpoint of safety because water is introduced in the proximity of molten aluminum alloy.